Anodized aluminium components are nowadays used in many fields in which, on the one hand, the decorative “aluminium look” is to be preserved and, on the other hand, corrosion protection of the sensitive aluminium surfaces are required. Fields which may be mentioned are the window and structural facings sector, or also the automotive field and domestic appliances.
Before the anodization, the aluminium parts are in most cases pickled in order to free the aluminium surfaces of impurities and remove the oxide layer (passive layer) that is naturally present. Depending on the type of treatment, semi-matte to matte-glossy surfaces are produced which, owing to their appearance, are referred to as so-called “satinized” surfaces.
In order to achieve improved corrosion protection, and also for design reasons, the aluminium components are anodized, that is to say an oxide layer is initiated by means of electrolysis. This method is also referred to as anodization, wherein aluminium is oxidized electrolytically. The aluminium surfaces so treated can also be dyed before final sealing.
The oxide layers produced by the anodization of aluminium and aluminium alloys are far superior to the oxide layers that occur naturally on the aluminium surfaces in terms of mechanical properties, corrosion resistance and decorative appearance. Good corrosion resistance to industrial and marine atmospheres is achieved with oxide layers of 20 μm. The oxide layer that is produced is not electrically conductive.
In order to be able to form homogeneous oxide layers, the anodization is preceded by mechanical or chemical pretreatment methods. These are specified, for example, in DIN 17611. Table 1 shows the designation system for the pretreatment of aluminium surfaces according to the mentioned DIN 17611.
TABLE 1Type ofSymbolpretreatmentNotesE0DegreasingSurface treatment before anodization, in which the surface andis degreased and deoxidized without further pretreatment.deoxidationMechanical surface flaws, e.g. pits and scratches, remainvisible. Areas of corrosion which were scarcely noticeablebefore the treatment can be visible after the treatment.E1GrindingGrinding leads to a comparatively uniform but slightlydull-matt appearance. All surface flaws present are largely removed, but grinding marks can remain visible.E2BrushingMechanical brushing produces a uniform glossy surface with visiblebrush strokes. Surface flaws are only partly removed.E3PolishingMechanical polishing leads to a glossy, bright surface, while surface flaws are only partly removed.E4Grinding andBy means of grinding and brushing, a uniformly glossy surface isbrushingachieved; mechanical surface flaws are removed. Areas of corrosionwhich can become visible with treatments E0 or E6 are removed. E5Grinding andBy means of grinding and polishing, a smooth, glossypolishingappearance is achieved; mechanical surface flaws areremoved. The effects of corrosion, which can become visible with treatments E0 or E6, are removed.E6PicklingAfter degreasing, the surface acquires a semi-matt or mattsheen by being treated with special alkaline picklingsolutions. Mechanical surface flaws are evened out but notremoved completely. The effects of corrosion on the metalsurface can become visible upon pickling. A mechanicalpretreatment prior to pickling can remove these effects;however, it is more beneficial to treat and store the metal in such a manner that corrosion is avoided.E7Chemical orAfter degreasing of the surface in a vapor degreasing electrochemicalagent or in a non-etching cleaning agent, a polishinghigh-gloss surface is obtained by treatment in specialchemical or electrochemical bright plating baths. Surface flaws are removed to only alimited degree,and effects of corrosion can become visible.E8Polishing andGrinding and polishing with subsequent chemical orchemical orelectrochemical polishing. This treatment leadselectrochemicalto a high-glossappearance; mechanical surface flaws polishingand the beginnings of corrosion are generally removed.
The mechanical, chemical and/or electrochemical pretreatments serve to prepare the surface of the aluminium substrates for the anodic oxidation. Specific surface effects can be achieved thereby. They serve to clean the surfaces of the components, to remove oxide layers (passive layer or incorrectly anodized surfaces) and surface defects. A uniform appearance of the surface is thereby achieved, and the resulting bright aluminium surface permits a brisk ion exchange during electrolysis. In addition, specific desired and also undesired structures are created, such as, for example, grinding and brush marks.
One of the chemical treatment methods that is frequently used is pickling of the aluminium substrate surfaces, which is also known as E6 treatment. Uniformly matte and decorative surfaces are thereby created; the so-called E6 finish. The irregularities on the surface that are produced by the extrusion of profiles and the rolling of metal sheets are to be covered or removed. Structural irregularities, such as web marks and welding seams, caused by the technology are also to be mattified as far as possible so that they are not troublesome from a decorative point of view.
Owing to the amphoteric properties of the aluminium surfaces, the component surface can be pickled both using bases and using acids. The oxide layer that is naturally present on the aluminium surface is thereby removed and any defects in the surface resulting from the production process are lessened. Consequently, a bright aluminium surface is obtained. This permits the very good ion exchange that is necessary for the subsequent electrolytic treatment.
In most cases, the aluminium substrates to be treated are introduced into an immersion bath containing an aqueous solution of sodium hydroxide (NaOH). Owing to its chemical properties, the sodium hydroxide solution serves both to remove impurities such as fats and oils and to pickle the aluminium surfaces. In many cases, however, cleaning and pickling take place separately, because a significantly lower NaOH concentration is sufficient for cleaning and thus, for example, less NaOH is carried into the subsequent rinsing tank. The cleaning process is referred to according to DIN17611 as E0. However, depending on the content of any alloying constituents of the substrate or on the effect to be achieved, further sodium compounds such as silicates, carbonates or phosphates can also be used.
Sodium oxide carries out a very strong attack on the oxides, oxide hydrates and the base metal of the aluminium substrate, which is about 20 times greater than at comparable acid concentrations. For an optimum reaction procedure, the dipping bath must be maintained in a defined temperature range. The temperature inside the pickling bath is dependent on the utilization over time. That is to say, the pickling bath must be heated if the supply of aluminium is low or, in the case of a very high aluminium supply, must also be cooled on account of the increased exothermic reaction in order to be kept in the optimum temperature range.
By the use of sodium hydroxide solution, the oxides and oxide hydrates are reacted as follows:AlO(OH)+NaOHNaAlO2+H2OAlO(OH)+NaOH+H2ONa[Al(OH)4]Al(OH)3+NaOHNa[Al(OH)4]Al2O3+2NaOH+3H2O2Na[Al(OH)4]
In addition, the lye also attacks the base metal:2Al+2H2O+2NaOH→2NaAlO2+3H2↑2Al+6H2O+2NaOH→2NaAl(OH)4+3H2↑
That is to say, sodium aluminate (NaAl(OH)4) and hydrogen (H2) form as reaction products. By adding additives such as, for example, nitrates or nitrites to the pickling medium, the evolution of hydrogen can be inhibited and the pickling operation accelerated, as a result of which the base metal is attacked to a lesser degree. However, these additives can lead to the formation of further critical waste products, such as, for example, ammonia, which pollute the waste water.
Furthermore, aluminium hydroxide can be deposited in the tank on the walls and the heating elements, so-called scale formation, which can hinder operation and in particular significantly impair the efficiency of the heating elements. Scale formation can be counteracted by adding complexing agents such as gluconates or phosphonates. However, such complexing agents are also environmentally relevant and can pollute the waste water.
Accordingly, depending on the particular process technique, the pickling process produces various types of waste, which are disposed of or treated in different ways. The consumed pickle and the resulting sludge must be disposed of as waste, which gives rise to disposal costs. The sludge produced from the pickle is usually deposited, which is associated with further disposal costs. Together, these disposal costs represent a considerable cost factor in the surface treatment of aluminium substrates.
In addition, the attack of the pickle on the aluminium surface to produce a so-called E6 finish also brings about the removal of an amount of material of up to 100 g/m2 or 30 μm. The attack and the removal of material thereby take place not only on the outside faces with a decorative requirement, but also on the inside faces, which remain invisible. In total, the amount of material removed is therefore up to 200 g/m2 or 60 μm of the total profile. The consumption of pickling agent is thereby linearly dependent on the amount removed. The loss of pickling agent by the removal of material that is caused can be compensated for relatively easily by adding fresh pickle. However, the sodium aluminate complex that forms also causes an increase in the viscosity of the pickle, as a result of which the pickling process becomes less effective over time. As the aluminium concentration increases, aluminium hydroxide also precipitates from the pickling medium and settles as a sludge-like deposit in the pickling tank. The pickling bath must therefore be renewed or replaced at regular intervals. Methods according to the current prior art are accordingly associated with a high occurrence of environmentally critical substances and a high energy consumption.